The invention relates to an optical disc comprising a synthetic resin substrate provided with readable information, a moisture-proof layer and a second synthetic resin layer, said moisture-proof layer being situated between the synthetic resin substrate and the second synthetic resin layer.
Such optical discs are known per se, in particular from, for example, U.S. Pat. Nos. 5,490,131 and 6,051,298. A compact disc (hereinafter referred to as CD) and a digital video disc (hereinafter referred to as a DVD) are examples of an optical disc which can be considered to be a customary optical information medium. The commercially available optical disc comprises a synthetic resin substrate which is successively provided with holes wherein information is stored, a reflection film covering these holes of the substrate of the disc, and a protective film formed on the reflection film, which reflection film can be considered to be a moisture-proof layer. More particularly, a CD is composed, in practice, of an approximately 0.05 micrometer thick reflection film covering the holes in the approximately 1.2 mm thick substrate of the disc, the protective film being provided on the reflection film in a thickness of approximately 10 micrometers. A DVD for a double-sided information system comprises a disc-shaped substrate wherein the information is stored in the form of pits, which are covered by a reflection film on which, in certain embodiments, a protective film is provided. Such a DVD can be obtained, for example, by arranging two of said discs on top of each other, said two discs being bonded together on the side of the protective films by means of an adhesive layer. In another embodiment of a DVD, there is one information and reflection layer.
The method of manufacturing optical discs is known per se and employs certain processes, such as compression, injection, photopolymerization, and the like, the disc-shaped substrate generally being made from polycarbonate. The moisture-proof layer covering the holes in the disc-shaped substrate is obtained by means of, for example, vacuum evaporation or sputtering, and is customarily made from aluminum. The second synthetic resin layer serving as a protective film is applied in the form of a layer to the aluminum moisture-proof layer in order to protect this thin aluminum film against damage that may result from general physical contact and from oxidation of the reflection film. This protective layer is generally provided by means of a spin-coating process wherein a resin material is applied through a rotatory movement. Such optical discs are generally employed in an environment where the atmospheric humidity is subject to variation. Thus, the water vapor or moisture present in the ambient air will be able to penetrate the comparatively dry synthetic resin substrate and, as a result of the fluctuation of the humidity in the surrounding atmosphere, diffuse from said substrate until an equilibrium situation is reached. As a result of the presence of the moisture-proof layer, penetration of moisture from the side of the protective film into the synthetic resin substrate will hardly take place. Instead, moisture can only penetrate into or desorb from the substrate on the other side of the substrate, i.e. the side of the substrate where the light is incident, if the optical disc in question is a CD-type optical disc. In the case of a DVR-type optical disc, where the second synthetic resin layer is the side on which the light is incident, the above-mentioned situation will be the other way round. Absorption or desorption of moisture leads to differences in water concentration, thereby causing volume-change differences of the substrate causing warpage of said substrate. In the embodiment of the optical disc comprising a synthetic resin substrate, the substrate thus provided with an angle of curvature is positioned substantially obliquely with respect to the optical axis of the incident light ray. As a result, the light ray will not travel along the center of a groove, not even when the servomechanism is in operation, resulting in a deterioration of the signal quality. Besides, in the case of further warpage of the substrate, the servomechanism would become unsuitable, thereby rendering the disc temporarily unusable. Thus, an optical disc which is subject to warpage beyond the requirements imposed on the radial angle is temporarily unreadable.
It is an object of the invention to provide an optical disc which exhibits no, or little, warpage under the influence of moisture.
The invention, as mentioned in the opening paragraph, is characterized in accordance with the invention in that the location of the moisture-proof layer in the optical disc is such that the following general formula applies:
0.14 less than xcex8 less than 0.49
wherein:   θ  =            thickness      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      the      ⁢              xe2x80x83            ⁢      second      ⁢              xe2x80x83            ⁢      synthetic      ⁢              xe2x80x83            ⁢      resin      ⁢              xe2x80x83            ⁢              layer        ⁢                              xe2x80x83                    ⁢                      xe2x80x83                          (        mm        )                    overall      ⁢              xe2x80x83            ⁢      thickness      ⁢              xe2x80x83            ⁢      optical      ⁢              xe2x80x83            ⁢      disc      ⁢              xe2x80x83            ⁢              (        mm        )            
It is to be noted that the term xe2x80x9cmoisture-proof layerxe2x80x9d used in the opening paragraph does not only comprise a single layer but also a so-called stack or packet of such layers, for example of metal, ZnSxe2x80x94SiO2, phase-change material and ZnSxe2x80x94SiO2. In the opening paragraph, such a stack or layer packet is to be considered to be a moisture-proof layer. In addition, it is possible that, in certain embodiments, the optical disc comprises two readable information layers, which are separated from each other by a so-called spacer. It will be clear that such an embodiment falls under the scope of protection of the invention, wherein these two information layers are to be considered to be a composite information layer.
The inventor has scrutinized the sensitivity of optical discs with respect to warpage caused by changes in the moisture content, and has surprisingly found that, if an optical disc meets the above-mentioned general equation, the angle of curvature of an optical disc can be minimized by the correct choice of the location of the moisture-proof layer in the optical disc.
It is particularly preferable for the location of the moisture-proof layer in the optical disc to be chosen such that the following general formula is met:
0.16 less than xcex8 less than 0.33
By suitably choosing the value of xcex8, an optical disc is obtained wherein the positive and the negative moment through, respectively, the substrate and the second synthetic resin layer largely compensate each other.